1. Field of the Invention
The present invention relates to an LSI internal signal observing circuit and, for example, to an LSI internal signal observing circuit which can easily observe a change in high-speed signal in an LSI at a pad of the LSI.
2. Related Art
As one of test methods which improve failure detection rate of an LSI (Large-Scale Integrated circuit), a method of improving observability to make it possible that an output in the circuit in test can be monitored outside the LSI. In order to perform such monitoring, there is proposed a test facilitating circuit in which a multiplexer and an output buffer for test are arranged to observe a desired internal signal in the LSI via a pad for test (for example, see Japanese Patent Laid-Open No. 10-107211 (page 3 and FIG. 1).
However, with an increase in speed of an internal signal, a normal output signal does not have been able to obtained from a conventional output buffer, because the conventional output buffer outputs a signal having an amplitude which is so large to follow a high-speed operation. As one of methods which avoid the problem, a method of using an output circuit of a small signal differential amplitude (LVDS: Low-Voltage Differential Signaling) scheme which can operate an output signal with a small amplitude is proposed (for example, see Japanese Patent Laid-Open No. 2000-134082 (pp. 3 to 4 and FIG. 1)).
However, the output circuit of the LVDS scheme disadvantageously has a complex circuit configuration and a large circuit scale. In addition, use of the special output circuit in only a test deteriorates efficiency in the use of the chip area of an LSI.